Brush and brush material

ABSTRACT

Brush material and brushes utilizing the same, particularly power driven rotary brushes, in which the brush bristles have a central core and a thin outer plastic coating, with granular abrasive preferably secured to such core beneath such coating.

mwQDmPxm Patented May 11, 1971 INVENTORS VERNON K. CHARVAT ROBERT E. JARl/l ATTORNEYS BY 3mm 0mm ED E v.25. mmszmm United States Patent 2,609,642 9/ 1952 Peterson Vernon K. Charvat Bay Village;

Robert E. Jarvi, Judson, Ohio 740,848

June 27, 1968 May 1 l, 1971 The Sherwin-Williams Company Inventors Appl. No. Filed Patented Assignee BRUSH AND mwsu MATERIAL 18 Claims, 1 Drawing Figs.

us. c1 15/119, 15/159, 51/298, 51/400 1111. c1 A4611 15/00 Field 01 Search... 15/159, 186-188, 200; 5 1/400-407, 295, 298; 161/174 References Cited UNITED STATES PATENTS Primary Examiner-Peter Feldman Attorney-Oberlin, Maky, Donnelly & Renner ABSTRACT: Brush material and brushes utilizing the same, particularly power driven rotary brushes, in which the brush bristles have a central core and a thin outer plastic coating, with granular abrasive preferably secured to such core beneath such coating.

BRUSH AND BRUSH MATERIAL This application relates as indicated to brush bristle material and more particularly to plastic-coated brush material of the general type disclosed in Peterson, U.S. Pat. No. 2,682,734. Such brush material is particularly suitable for use in power driven rotary brushes, such as wheel brushes, end brushes, cup brushes, and the like.

It has also been known in the past to employ plastic brush bristle material with abrasive grains incorporated therein as taught by Radford, U.S. Pat. No. 2,328,998, for example, but brushes employing such latter type brush bristle material have not received wide acceptance for a variety of reasons, including difficulty of manufacture and the fact that a large portion of the abrasive grains are so embedded and submerged in the plastic material as to be relatively ineffective when applied to the work. The method of manufacture taught by Radford also tends to result in excessive erosion of the extruder parts.

It is accordingly an important object of the present invention to provide a novel brush bristle material having a central filamentous core with granular abrasive bonded to the exterior of such core, and a thin outer plastic coating conforming generally to the abrasive grains to produce an irregular outer surface of the composite bristle.

Another object is to provide brushes, and especially power driven rotary brushes, utilizing such new brush material.

Still another object is to provide composite brush bristle material having good tensile strength as well as good compressive strength and internal adhesion of the components.

A.still further object is to provide a method for producing such brush material in continuous lengths which may thereu pon be cut to the desired bristle length.

Other objects, features and advantages of this invention will become apparent to those skilled in the art after a reading of the follom'ng detailed description.

To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter additionally described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

In said annexed drawings:

FIG. 1 is a diagrammatic side elevational view, partially in cross section, of certain preferred apparatus for employing in the continuous manufacture of abrasive bristle material in accordance with this invention;

FIG. 2 is a much enlarged side view of a portion of a brush bristle in accordance with the invention, with a portion of the thin outer coating removed better to disclose the internal construction thereof;

FIG. 3 is a transverse cross section through such bristles where the latter emerges from the die of the extruder of FIG. 1, indicating generally the manner in which the outer plastic coating is applied;

FIG. 4 is a side view of a typical wheel-type power driven rotary brush utilizing such new brush bristle material therein;

FIG. 5 is a side view of the brush of FIG. 4 wherein such brush bristle material is further embedded in an elastorneric matrix body;

FIG. 6 is a section taken on the line 6-6 on FIG. 4; and

FIG. 7 is a transverse cross section of another embodiment of the invention similar to FIG. 3 but showing a multiple core bristle on a much enlarged scale.

Referring now more particularly to said drawing, and especially to FIG. 1 thereof, a continuous strand 1 of fine glass monofilaments is withdrawn from a supply reel 2 under back tension and passed around grooved ceramic pulleys 3, 4, 5 and 6,.to'coat the strand with primer or binder liquid 7 contained in primer tank 8. Such strand, with a thin coat of primer thereon, is. next conducted through a fluidized bed of frne granular abrasive material within enclosure 9, such bed being thus continuously fluidized by means of air jets from perforated manifold 10 in well known manner. This serves to adhere such abrasive grains at closely spaced intervals on the binder coated surface of strand 1. By regulating the air jets it is possible to elevate the fluidized bed in the middle just sufficiently (usually about one-fourth to about one-half inch) to pass the coated strand through only the extreme upper portion of the bed, thereby gently to deposit the abrasive thereon instead of abrading such strand. This novel technique is also useful for the application of granular material to traveling sheets or tapes of paper or cloth, in the production of sandpaper and the like. Subsequently such abrasive coated strand passes through oven ll heated by element 12 to set the binder so that the abrasive grains are now securely bonded in place.

At the next station, the abrasive coated strand passes through a low-pressure extruder 13 where a thin plastic outer, coating 14 is applied, the die 15 having a die opening '16 (see FIG. 3) of sufficient size that only such thin outer plastic coating is applied to the rapidly travelling strand without appreciable abrading action of the die by the bonded abrasive grains 17.

In effect, the rapidly travelling strand passing through the die draws a very thin coating of the plastic onto its surface rather than having an extruded layer applied thereto conforming to the shape of the die opening. Nylon is preferred coating material for this purpose. The plastic coated strand will then normally be passed around pulleys l8, 19, 20 and 21 to conduct the same through a water bath 22 in tank 23 before being wound uponpower driven takeup reel 24. The speed of strand travel will normally be between about 300 and 1,100 feet per minute.

The openings in the cover of fluid bed 9 and oven 11 through which the strand rapidly passes are sufficiently large that such strand will not contact the edges thereof, such strand, of course, being under tension, and the strand likewise passes through extruder 13 with sufficient clearance not to abrade the latter. As indicated above, the fluid plastic coating material 14 applied to the travelling strand by such extruder is of sufficient viscosity, and the rate of travel of the strand is sufficiently high, that only a very thin plastic coating is applied as the strand emerges from relatively large die opening 16. The entrance side of the extruder may be kept under a very small vacuum to enhance the tightness of the nylon coating.

The resultant brush material 25, illustrated in FIGS. 2 and 3, may accordingly comprise a central core of glass fiber monofilaments I having a thin inner coating or layer of binder 7 to which the grains 17 of abrasive are adhered, with thethin outer plastic coating 14 overlying such grains and binder. This new brush material may then be cut to any desired length and utilized in the manufacture of brushes, especially power driven rotary brushes, such as those disclosed in Whittle, U.S. Pat. No. 2,288,337, for example. As shown in FIG. 4-of the drawing, the brush bristles 25 may thus be retained in a sheet metal channel-form back or hub 26 with the bristles extending generally radially outwardly therefrom. Such bristles will normally be straight and have been found to be especially useful in the manufacture of short trim brushes; that is to say, brushes in which the bristles are relatively short, on the order of 2 inches, for example, in a 12 inch OD. 8 inch I.D. wheel type brush, with one-half inch of the bristle length retained within the brush back and 15g inches extending therefrom.

In view of the .thinness of the outer plastic coating 14, smallhumps orbumps 27 appear at closely spaced intervals on the outer surface of the new composite bristle where such coating overlies the individual abrasive grains, rendering such bristle somewhat less slippery to handle than the usual plastic coated filament. This is beneficial in the brush manufacturing opera-.

tions where such bristles, cut to relatively short length, must be properly and uniformly distributed within the brush back or equivalent holder member. In the past, this has frequently posed a practical problem inasmuch as the bristles have tended to shift relative to each other and to the back of the brush, with consequent nonunifonn distribution circumferentially of the back as well as resulting ingeneral disarray of the bristles. Such small humps or protuberances also serve somewhat to modify and regulate interaction of the bristles when the brush is put into use, assisting in avoiding concentrations of stress in the bristles as they flex in engagement with the work. A bulking effect is achieved and a more uniform brush face provided.

A wide variety of fine filamentous material may be employed as the core, and, in some instances, such core may even comprise a single relatively large monofilaments. Ordinarily, however, astrand or bundle of fine filaments, such as glass fibers, is preferred. The initial strand 1 as it comes from the supply reel may thus comprise, for example, about 600 fine glass fiber monofilaments lightly adhered together with starch and oil. A very satisfactory material for this purpose is Owens- Coming ECG 150%glass fiber, this being a plied strand of continuous electrical glass monofilaments which are 0.00036 inch diameter and 15,000 yards/lb.

The individual glass monofilaments comprising the strand will ordinarily be from 0.0002] inches to 0.00060 inches in diameter, approximately 0.00025 inches being preferred. The filaments may be parallel, twisted or braided but generally parallel filaments are preferred and the strand should preferably have a size of from about 15,000 yards/lb. to about 45,000 yards/lb. Not only does the glass fiber core provide tensile strength and contribute to the brushing effectiveness of the finished bristle; it also withstands the temperature of the nylon extruder and permits the application of substantial tension during the manufacturing operation. Other materials such as polypropylene yarn, or fine hard wire filaments, or mixtures of these with each other or with glass fibers, may also be employed.

The binder or primer 7 is preferably polyvinyl acetate in ethanol, but other binders such as epoxy solutions may be employed. The polyvinyl acetate in ethanol binder comprises about 25 percent of solids and a die at the exit end of the primer tank 8 serves to wipe the travelling strand to leave a coating of about 0.005 inch thickness; upon drying, such coating is about 0.001 -inch thick, thus adding about 0.002 inch to the diameter of the strand. A suitable epoxy binder is 8-7960- l Epoxy Coating supplied by Mobil Chemical Company. Polyurethane is likewise a good primer, such as Polane," obtainable from Lowe Brothers, Dayton, Ohio.

If the binder coating is relatively thick it is possible slightly to fracture the same after drying by heating or mechanical flexing and thereby loosen small flakes of the binder to produce additional small random protuberances on the surface which are covered by the thin nylon coating similarly to the abrasive grains, further roughing the surface of the finished bristle material and enhancing its handling qualities.

The fine granular abrasive which is adhered to the binder coated strand as the latter passes through the fluid bed may be any suitable abrasive selected with the ultimate brush operation in mind, such as aluminum oxide, silicone carbide, chrome oxide, pumice or emery, but abrasives are preferred which occur in the form of elongated rough-surfaced grains rather than those which ordinarily occur as smooth blocky grains. For this reason, silicon carbide is a much preferred abrasive for employment in accordance with the invention and may be used in the range of 80-500 mesh, although 120- -380 mesh is preferred. The elongated silicon carbide crystals (frequently about three times as long as wide) become aligned parallel to the bristle which enhances their effectiveness when applied to the work, A relatively light deposit of the abrasive is preferable to a substantially continuous coating of abrasive on the strand and the grains may be distributed approximately one grain diameter apart thereon. The abrasive content including both the grain and glass fibers, may comprise from percent to 45 percent of the finished product by volume, with from 25 percent to 35 percent by volume being preferred.

Similarly, a variety of outer plastic coatings may be employed, with nylon (polyamide resins) preferred. Foamed polyurethane and polypropylene are also excellent, and other materials such as the vinyls (vinyl polymers and copolymers) and trifluorochloroethylene polymer may be employed. When employing nylon, the standard type of 6/6 heat-stabilized nylon is preferred. The extruder is maintained at 300 C. when applying such molten nylon to the strand and the applied coating is then cooled to 1 30 C. in water bath 22.

In order further to enhance the handling characteristics of the new brush bristle material, the travelling strand may be conducted through a fluidized bath of fine mica (325-60 Omesh, 400 being preferred) or like material immediately following passage of such strand through the nylon extruder. The mica thus adheres to the nylon coating which is still molten or soft. This treatment has been found to be advantageous for plastic and plastic coated brush bristles even when no abrasive grains are incorporated in the latter. The mica coated bristle material is much more easily handled in the brush manufacturing procedures and the action of the bristles in use is also somewhat modified as a result of this coating. Brush bristle material of this type has been found to be particularly useful and advantageous in the manufacture of brush strip, and especially of brush strip which is wound in helical conformation such as that disclosed in Peterson, US. Pat. No. 2,303,386.

A further modification which has been found useful in the manufacture of plastic coated brush bristle materials is the incorporation of very finely chopped glass fibers in the outer plastic (e.g. nylon) coating. Such glass fibers may desirably be about one-sixteenth to one-eighth inch long. They serve to strengthen and stiffen the bristle, reinforce the outer plastic coating against rupture when the bristle is flexed, and they also provide some additional abrasive action on the work.

Instead of employing the primer 7, it is also possible to apply the outer nylon or other plastic coating by first removing the usual starch and oil binder from the glass fiber strand or bundle and chemically applying a negative charge to the fibers to fluff the bundle, as by means of Tamol" (sodium salt of condensed sulfonic acid) supplied by Rohm & Haas Co. The nylon is thus enabled to penetrate the bundle somewhat and thereby adhere thereto. Alternatively, similar pretreatment of the strand enhances penetration and bonding of the primer coating. A small amount of abrasive grain, approximately 7 to 9 percent by volume of the finished bristle material may be incorporated in the nylon and a very small amount, on the order of 1 percent by volume, may also be adhered to the soft nylon coating as the strand exits from the extruder.

it has also been found to be advantageous in the case of the nylon coated strand, whether including the abrasive grains or not, to bake the finished brush filled with such brush material 7 at from about 300 F. to about 375 F. (preferably approximately 325 F.) for approximately 2 hours to increase the bond between the nylon coating and the core. The bristles will normally shrink approximately 0.002 inches in diameter under this treatment and become more uniform, stabilizing the brush. It has been found that a power driven brush having nylon coated brush bristle material treated in this manner will hold together very much better in use, particularly under wet operating conditions. The treatment tends to crystallize and orient the nylon coating and to set" the bristles 25 in their hairpin shape where retained within the brush back by element 29 so that they are held more securely (FIG. 6). This is in contrast to what might be expected, in that loosening of the bristles due to such shrinkage would normally be anticipated. However, the setting of the bristles in their hairpin form eliminates the natural tendency to straighten which in the past has sometimes resulted in a certain amount of loosening and shedding. The sheet metal channel-form back 26 may subsequently be compressed, if desired, but this has not been found necessary in practice.

The final bristle material of this invention comprising the central filamentous core, binder layer, abrasive, and outer plastic coating will ordinarily be from about 0.030 to 0.035 inch in diameter. The glass fiber core may constitute about 30 percent the primer 5 percent, the abrasive grains 25 percent, and the outer plastic coating about 35 percent by volume, of the finished product. However, for some special purposes, a bigger strand of core filaments and two coatings of abrasive grains may be employed, so that the finished bristle may be 0.070 or 0.075 inch in diameter. As noted above, the bristle material of this in invention is especially useful in short trim brushes which have a relatively stiff bristle action and accordingly apply the abrasive grains forcefully to the work. Although such short bristles are still, especially when the outer plastic coating is reinforced with chopped fiber and the core is straight, they nevertheless flex in use to afford a true, harsh, brushing action. They will ordinarily be as densely packed together as possible to provide a maximum of abrading points engaging the work. The brushing action may be further modified by providing somewhat larger plastic protuberances at spaced intervals along the bristles as taught in Charvat, US. Pat. No. 3,090,061. The brush utilizing the new bristle material of this invention may also be embedded in an elastomeric matrix (FIG. 5), preferably foamed polyurethane, although a number of other elastomeric material such as polyurethane rubber and foamed polychloroprene may also be used. Various filler materials may be incorporated in such matrix as explained in Peterson, US. Pat. No. 3,076,219 to ensure that the matrix erodes back slightly from the brush face in use, thereby to maintain the bristle end portions always projecting slightly from such matrix.

Referring now more particularly to FIG. 7 of the drawing, a plurality of glass fibers strands or bundles 1 may be coated with primer 7, the abrasive grains l7 adhered thereto, and then after drying in oven 11 brought closely together as shown, ordinarily with slight twisting, before application of the thin outer nylon coating 14. When employed in the production of such multiple core bristle material the individual glass fiber bundles will ordinarily be of somewhat smaller diameter than when only a single bundle is employed as in the FIG. 2 embodiment. The resultant bristles may thereafter be handled and treated in the same general manner described elsewhere herein, but the multiple core bristles are considerably stifier and more harsh in their brushing action on the work.

It is preferred that there be no abrasive grains in the central fiber core 1 of the finished bristle material but when a plurality (e.g. three) abrasive coated strands are plied together prior to application of the outer nylon coating there will then, of course, be a certain amount of abrasive internally of such finished composite bristle. Since the abrasive grains on the outer surface of the bristle are covered by only a very thin coating of nylon or other plastic material is has been found that such bristles flex more readily to afford the desired brushing action in use than is the case with bristles of the type disclosed in Radford, Pat. No. 2,328,998, for example. The bristle material also handles more easily during brush manufacture, may be packed more densely together in the brush back, and the mutual support of the bristles serves better to distribute the stresses imposed thereon during high speed operation of the brush. The nylon does itself readily adhere to the abrasive grains and the latter are accordingly effectively applied to the work where exposed at the working ends of the bristles. The nylon coating does, however, not only strengthen and waterproof the bristle, as well as assisting in retaining the grains in place, but also by covering such grains along the length of the bristle much reduces undesirable sawing action and wear between the bristles themselves.

When employing a bundle of glass fibers or filaments as the bristle core it is preferred that such core comprise at least about 30 percent by volume of the finished bristle. Surprisingly, superior results are obtained when the brush bristle contains only a very small amount of abrasive grains, about 8 percent to about l2percent by volume of the entire bristle. The abrasive grains are more effective in their application to the work in a brushing operation if they are well spaced apart on the bristle, obtaining a better cut and metal removal due to the increased unit pressures resulting when only relatively few grains per bristle engage the work at any one time. Furthermore, the bristles flex more readily as is desirable in a true brushing operation.

The nonskid properties of the bristles maybe enhanced by providing amorphous silica of small particle size (e.g. 4 microns average) in the water tank 23 before the takeup reel. A water dispersion of starch and silica prepared by mixing with a turbine agitator is effective. A very thin coating is thus applied to the bristle material adding only about one-tenth of 1 percent or less to the bristle weight.

The brushes of this invention are especially useful as flexible abrasive-applying tools for such operations as burr removal, surface and edge blending of parting lines on jet aircraft turbine blades, and surface finishing.

Other modes of applying the principles of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

We claim:

1. A brush comprising a brush bristle support and bristles extending therefrom, in which said bristles comprise an elongated inner core of a large number of fine glass filaments, abrasive grains adhered to the exterior of said core in slightly spaced apart relationship, said abrasive grains being bonded to the exterior of said core by a primer coating on said core and the interior of said core being free of said abrasive grains, and a thin outer nylon coating over said core and abrasive grains, said outer coating being sufficiently thin to conform to said grains sufficiently to form corresponding protuberances on the surface of the bristle, wherein said filamentous core comprises approximately 600 glass monofilaments, said abrasive grains are elongated silicon carbide crystals of to 500 mesh oriented generally parallel to the bristle length and spaced about 1 grain diameter apart on the average, and said outer nylon coating is heat-stabilized.

2. The brush of claim I, wherein said filamentous core constitutes about 30 percent, the primer coating constitutes about 5 percent, the abrasive grains constitute about 25 percent, and the outer nylon coating constitutes about 35 percent, all by volume, of the bristle, said bristles being about 0.030 to about 0.035 inch in diameter.

3. A brush comprising a brush bristle support and bristles extending therefrom, in which said bristles comprise an elongated inner core, abrasive grains adhered to the exterior of said core, and a thin outer plastic coating over said core and abrasive grains, said outer coating being reinforced with short fine fibers incorporating therein and being sufficiently thin to conform to said grains sufiiciently to form corresponding protuberances on the surface of the bristle.

4. The brush of claim 3, wherein said short fine fibers are chopped glass fibers about one-sixteenth to about one-eighth inch in length.

5. Brush bristle material comprising an inner filamentous core of a large number of fine glass filaments, abrasive grains bonded to the exterior of said core in slightly spaced apart relationship, and a thin outer plastic coating of-sufficient thinness and conforming to said grains sufficiently to form corresponding protuberanceson the surface of the bristle, said outer plastic coating beingof heat shrunk nylon.

6. The brush bristle material of claim 5, wherein said grains are rough surfaced, elongate, and arranged with their longest dimensions'generally parallel to the length of said bristle.

7. Brush bristle material comprising a plurality of individual strands of elongated material, each said strand comprising bundles of filaments and having abrasive grains adhered to the surfaces thereof, said strands being assembled in close side-byside relationship, and a thin outer coating of plastic-material covering the assembly of said strand and grains, said coating confonning to saidgrains sufficiently to form small protuberances on the surface of the finished composite bristle.

8. The bristle material of claim 7, wherein said strandsare bundles of glass fibers, said grains are bonded thereto in spaced relationship, and said thin outer coating is nylon.

9. Brush bristle material comprising an elongated strand having granular abrasive adhered thereto, and a thin outer plastic coating covering said strand and abrasive, wherein said granular abrasive comprises only from about 8 percent to 12 percent of the total volume of said bristle.

l0. Brush bristle material having elongate abrasive grains secured thereto and disposed with their longest dimension generally parallel to the length of said bristle, said grains being spaced apart.

11 A rotary brush comprising a central brush bristle support and bristle extending generally radially outwardly therefrom, which bristles are at least in part of heat shrunk and heat stabilized nylon.

12. The brush of claim 11, wherein said bristles comprise an inner filamentous core having a coating of heat shrunk and heat stabilized nylon thereon.

13. The brush of claim 12, wherein said core comprises fine glass fibers.

14. The brush of claim 12, wherein abrasive grains are adhered to said core under said coating.

15. The brush of claim 14, wherein said core comprises fine glass fibers, and said abrasive grains constitute only from about 8 percent to about 12 percent of the total volume of the bristle.

16. The brush of claim 14, wherein said coating is reinforced with a quantity of short fine fibers incorporated therein.

17. The brush of claim 12, wherein said bristles are embedded in a unitary body of elastomeric material from which only the extreme outer ends of said bristles protrude.

18. A rotary brush comprising a support and straight, stiff bristles extending therefrom, in which said bristles comprises an elongated inner core of fine glass fibers filaments, abrasive grains adhered to the exterior of said core which is free of said grains internally thereof, and a thin outer plastic coating over said core and grains, said coating being sufficiently thin to conform to said grains sufficiently to form corresponding protuberanccs on the surface of the bristle. 

1. A brush comprising a brush bristLe support and bristles extending therefrom, in which said bristles comprise an elongated inner core of a large number of fine glass filaments, abrasive grains adhered to the exterior of said core in slightly spaced apart relationship, said abrasive grains being bonded to the exterior of said core by a primer coating on said core and the interior of said core being free of said abrasive grains, and a thin outer nylon coating over said core and abrasive grains, said outer coating being sufficiently thin to conform to said grains sufficiently to form corresponding protuberances on the surface of the bristle, wherein said filamentous core comprises approximately 600 glass monofilaments, said abrasive grains are elongated silicon carbide crystals of 80 to 500 mesh oriented generally parallel to the bristle length and spaced about 1 grain diameter apart on the average, and said outer nylon coating is heat-stabilized.
 2. The brush of claim 1, wherein said filamentous core constitutes about 30 percent, the primer coating constitutes about 5 percent, the abrasive grains constitute about 25 percent, and the outer nylon coating constitutes about 35 percent, all by volume, of the bristle, said bristles being about 0.030 to about 0.035 inch in diameter.
 3. A brush comprising a brush bristle support and bristles extending therefrom, in which said bristles comprise an elongated inner core, abrasive grains adhered to the exterior of said core, and a thin outer plastic coating over said core and abrasive grains, said outer coating being reinforced with short fine fibers incorporating therein and being sufficiently thin to conform to said grains sufficiently to form corresponding protuberances on the surface of the bristle.
 4. The brush of claim 3, wherein said short fine fibers are chopped glass fibers about one-sixteenth to about one-eighth inch in length.
 5. Brush bristle material comprising an inner filamentous core of a large number of fine glass filaments, abrasive grains bonded to the exterior of said core in slightly spaced apart relationship, and a thin outer plastic coating of sufficient thinness and conforming to said grains sufficiently to form corresponding protuberances on the surface of the bristle, said outer plastic coating being of heat shrunk nylon.
 6. The brush bristle material of claim 5, wherein said grains are rough surfaced, elongate, and arranged with their longest dimensions generally parallel to the length of said bristle.
 7. Brush bristle material comprising a plurality of individual strands of elongated material, each said strand comprising bundles of filaments and having abrasive grains adhered to the surfaces thereof, said strands being assembled in close side-by-side relationship, and a thin outer coating of plastic material covering the assembly of said strand and grains, said coating conforming to said grains sufficiently to form small protuberances on the surface of the finished composite bristle.
 8. The bristle material of claim 7, wherein said strands are bundles of glass fibers, said grains are bonded thereto in spaced relationship, and said thin outer coating is nylon.
 9. Brush bristle material comprising an elongated strand having granular abrasive adhered thereto, and a thin outer plastic coating covering said strand and abrasive, wherein said granular abrasive comprises only from about 8 percent to 12 percent of the total volume of said bristle.
 10. Brush bristle material having elongate abrasive grains secured thereto and disposed with their longest dimension generally parallel to the length of said bristle, said grains being spaced apart. 11 A rotary brush comprising a central brush bristle support and bristle extending generally radially outwardly therefrom, which bristles are at least in part of heat shrunk and heat stabilized nylon.
 12. The brush of claim 11, wherein said bristles comprise an inner filamentous core having a coating of heat shrunk anD heat stabilized nylon thereon.
 13. The brush of claim 12, wherein said core comprises fine glass fibers.
 14. The brush of claim 12, wherein abrasive grains are adhered to said core under said coating.
 15. The brush of claim 14, wherein said core comprises fine glass fibers, and said abrasive grains constitute only from about 8 percent to about 12 percent of the total volume of the bristle.
 16. The brush of claim 14, wherein said coating is reinforced with a quantity of short fine fibers incorporated therein.
 17. The brush of claim 12, wherein said bristles are embedded in a unitary body of elastomeric material from which only the extreme outer ends of said bristles protrude.
 18. A rotary brush comprising a support and straight, stiff bristles extending therefrom, in which said bristles comprises an elongated inner core of fine glass fibers filaments, abrasive grains adhered to the exterior of said core which is free of said grains internally thereof, and a thin outer plastic coating over said core and grains, said coating being sufficiently thin to conform to said grains sufficiently to form corresponding protuberances on the surface of the bristle. 